Spectral Control System

ABSTRACT

The disclosure is directed to a system and method of controlling spectral attributes of illumination. According to various embodiments, a portion of illumination including an excluded selection of illumination spectra is blocked, while another portion of the illumination including a transmitted selection of illumination spectra is directed along an illumination path. In some embodiments, optical metrology is performed utilizing the spectrally controlled illumination to enhance measurement capability. For instance, the spectral attributes of illumination utilized to analyze different portions of a sample, such as different semiconductor layers, may be selected according to certain measurement characteristics associated with the analyzed portions of the sample.

PRIORITY

The present application claims priority to U.S. Non-Provisionalapplication Ser. No. 13/945,352, entitled SPECTRAL CONTROL SYSTEM, ByAmnon Manassen, Andrew V. Hill, Ohad Bachar, Avi Abramov, and DaniaNegri, filed Jul. 18, 2013, which claims priority to U.S. ProvisionalApplication Ser. No. 61/738,322, entitled SPECTRUM FLEXIBILITY OFOPTICAL METROLOGY, By Amnon Manassen et al., filed Dec. 17, 2012, andU.S. Provisional Application Ser. No. 61/808,555, entitled SPECTRUMFLEXIBILITY OF OPTICAL METROLOGY, By Amnon Manassen et al., filed Apr.4, 2013. The foregoing non-provisional and provisional applications arehereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure generally relates to the field of optical systemsand more particularly to an optical system for controlling spectralattributes of illumination.

BACKGROUND

Modern process control targets are subject to process developmentsdesigned for smaller features. For instance, optical metrology targetsare being fabricated with thinner layers and materials with differentoptical constants. Developments in target design and other factorsincrease measurement sensitivity to the spectral content of probingbeams. Accordingly, spectrally controlled illumination is often employedto enable monitoring and control of the effect of wavelength onmeasurement.

In some imaging and angular scatterometry systems, spectral bands areselected using filters. However, a bandwidth of a few tens of nanometersis required in order to maintain adequate precision*MAM performance.Spectroscopic scatterometers often include spectrometers for detectionof illumination spectra. Imaging and angular scatterometry suffer from alimited choice of spectral structures which limits measurement potentialbecause of the limited number of wavelength structures available pertarget. Similar limitations apply for spectroscopic scatterometry,because of the limited number of angular structures available forselection. There is a need in the art for increased configurability inspectral control systems.

SUMMARY

In one aspect, the disclosure is directed to a spectral control systemand associated method, where the spectral control system includes atleast a dispersion path, a spectral controller, and a combination path.One or more dispersive elements disposed along a dispersion path areconfigured to receive at least a first portion of illumination directedalong the dispersion path from at least one illumination source. Thedispersive elements are further configured to disperse the first portionof the illumination into a first plurality of dispersed portions ofillumination. At least one spectral controller is configured to receivethe first plurality of dispersed portions of illumination from thedispersion path. The spectral controller is further configured to blockan excluded selection of the first plurality of dispersed portions ofillumination and direct a transmitted selection of the first pluralityof dispersed portions of illumination along the combination path. One ormore combination elements disposed along the combination path areconfigured to combine the transmitted selection of the first pluralityof dispersed portions of illumination into substantially coaxialillumination directed along an illumination path.

In another aspect, the disclosure is directed to a spectral controlsystem and associated method, where the spectral control system includesa plurality of spectral control paths and at least one optical switch.At least a first optical switch is configured to direct illuminationemanating from at least one illumination source along a selectedspectral control path. Each spectral control path may be configuredaccording to selected spectral attributes. For instance, a firstspectral control path may be configured to filter out a first selectionof illumination spectra, and a second spectral control path may beconfigured to filter out a second (different) selection of illuminationspectra. The optical switch may allow rapid spectral control byswitching from one preconfigured spectral control path to another. Thespectrally controlled illumination from the selected path is directedalong an illumination path. In some embodiments, the system furtherincludes a second optical switch working in parallel with the firstoptical switch to direct spectrally controlled illumination receivedfrom the selected spectral control path along the illumination path.

In another aspect, the disclosure is directed to an optical metrologysystem and associated method, where the optical metrology systemincludes a spectral control system, such as the spectral control systemdescribed in at least one of the preceding paragraphs or as furtherdescribed below. The optical metrology system may further include atleast one illumination source, an optical measurement head, at least onedetector, and at least one computing system communicatively coupled tothe detector. The illumination source is configured to provideillumination along an optical path to the spectral control system. Theoptical measurement head is configured to illuminate a sample disposedupon a sample stage utilizing at least a portion of (spectrallycontrolled) illumination received from the illumination path of thespectral control system. The detector is configured to receiveillumination scattered, reflected, or radiated from the sample. Thecomputing system is configured to determine at least one spatialattribute of the sample based upon the illumination scattered,reflected, or radiated from the sample.

The spectral control system may enable the optical measurement head toilluminate a first portion of the sample with a first portion ofillumination including a first selection of illumination spectra, asecond portion of the sample with a second portion of illuminationincluding a second selection of illumination spectra, and so on. In someembodiments, different layers of a sample are thus analyzed usingillumination with selected spectral attributes for enhanced measurementcapability.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not necessarily restrictive of the present disclosure. Theaccompanying drawings, which are incorporated in and constitute a partof the specification, illustrate subject matter of the disclosure.Together, the descriptions and the drawings serve to explain theprinciples of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The numerous advantages of the disclosure may be better understood bythose skilled in the art by reference to the accompanying figures inwhich:

FIG. 1A is a block diagram illustrating a system for controllingspectral attributes of illumination, in accordance with an embodiment ofthis disclosure;

FIG. 1B is a block diagram illustrating the spectral control system,wherein dispersive elements of the system include optical prisms, inaccordance with an embodiment of this disclosure;

FIG. 1C is a view of illumination dispersed across a surface of aspectral controller of the spectral control system, in accordance withan embodiment of this disclosure;

FIG. 2A is a block diagram illustrating a multi-path system forcontrolling spectral attributes of illumination, wherein the systemincludes a plurality of dichroic splitters and a plurality of dispersiveelements for multi-path dispersion of selected spectral bands, inaccordance with an embodiment of this disclosure;

FIG. 2B is a view of illumination from the selected spectral bandsdispersed across a surface of a spectral controller of the multi-pathspectral control system, in accordance with an embodiment of thisdisclosure;

FIG. 3 is a block diagram illustrating a system for controlling spectralattributes of illumination, in accordance with an embodiment of thisdisclosure;

FIG. 4 is a block diagram illustrating an optical metrology systemincluding a spectral control system, in accordance with an embodiment ofthis disclosure;

FIG. 5 is a flow diagram illustrating a method of controlling spectralattributes of illumination, in accordance with an embodiment of thisdisclosure;

FIG. 6 is a flow diagram illustrating a method of controlling spectralattributes of illumination, in accordance with an embodiment of thisdisclosure; and

FIG. 7 is a flow diagram illustrating a method of performing opticalmetrology, in accordance with an embodiment of this disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to the subject matter disclosed,which is illustrated in the accompanying drawings.

FIGS. 1A through 7 generally illustrate a system and method forcontrolling spectral attributes of illumination to improve opticalmetrology results. According to various aspects of the disclosure,spectral structures may be selected or configured using a spectralcontrol system to achieve specified levels of contrast, performance, andaccuracy. As angular and spectral scatterometry are two faces of Braggdiffraction, it is advantageous to measure and control angle andspectrum for improved performance. Spectroscopic scatterometry resultshave shown that sensitivity can be achieved over a range of many tens ofnanometers. Accordingly, a spectral structure including selectedsensitive portions of the spectra will lead to improved results.Further, angular scatterometry measurements may be taken with a selectedspectral structure without laser coherence effects on one side andlimited precision*MAM penalty when using simple filters on the otherside. In spectroscopic scatterometry, the ability to select a spectralstructure may further enable replacement of a spectrometer with a simpledetector by integrating the signal from a predefined spectrum.

FIGS. 1A through 1C illustrate embodiments of a spectral control system100 including a dispersion path with one or more dispersive elements 104and a combination path with one or more combination elements 108. Thesystem 100 is configured to receive illumination directed along thedispersion path by at least one illumination source 102, such as a laserdriven light source (LDLS), a laser sustained plasma (LSP), or any otherbroadband illuminator. The one or more dispersive elements 104 mayinclude diffraction gratings or prisms (illustrated in FIG. 1B)configured to disperse at least a portion of the illumination directedalong the dispersion path into a plurality of dispersed portions ofillumination. For example, at least a first dispersive element 104 maybe configured to disperse a first portion of illumination defined by afirst spectral band or range.

The resulting dispersed portions of illumination may appear as aspectral continuum, as shown in FIG. 1C. The dispersed portions ofillumination are directed from the dispersion path to at least onespectral controller 106, such as a mirror array (e.g. DLP micro-mirrorarray), a plurality of active shutters, a plurality of selectablefilters or masks, or any other spatial light modulator (SLM) known tothe art. The spectral controller 106 may be configured to block or stopa selection of the dispersed portions of illumination, thereby excludingthe corresponding illumination spectra. The spectral controller 106 maybe further configured to transmit or direct the remaining portions ofdispersed illumination along the combination path. The one or morecombination elements 108 may include a second set of diffractiongratings or prisms configured to recombine the transmitted selection ofdispersed illumination into substantially coaxial (i.e. undispersed)illumination. The recombined spectrally controlled illumination may thenbe directed along an illumination path to an output source, such as anoptical measurement head of an optical metrology system, such as anangular or spectroscopic scatterometry metrology system.

The various optical “paths” described herein may be delineated by aplurality of optical elements, such as one or more of: focusing lenses,coupling lenses, polarizers, beam splitters/combiners, optical fibers,dispersive elements, combination elements, and the like. For example, asillustrated in FIGS. 1A and 1B, the illumination path may include acoupling lens configured to direct the spectrally controlledillumination into an optical fiber 110 (e.g. A500 square-core fiber) fordelivery to a selected output source. In some embodiments, for example,an optical measurement head of an optical metrology system may includethe output end of the optical fiber 110 or may be configured to receivespectrally controlled illumination from the output end of the opticalfiber 110.

According to various embodiments, the spectral controller 106 mayinclude or may be driven by at least one computing system. Further, itshould be recognized that the various steps and functions describedthroughout the present disclosure may be carried out by a singlecomputing system or by multiple computing systems. For example, the oneor more computing systems may include, but is not limited to, a personalcomputing system, mainframe computing system, workstation, imagecomputer, parallel processor, or any other device known in the art. Ingeneral, the term “computing system” may refer to at least onesingle-core or multiple-core processor configured to execute programinstructions from communicatively coupled carrier media.

Further, the spectral controller 106 may include one or more actuatorsdriven by the computing system to configure an array of mirrors, openand close active shutters, rotate a filter wheel to select anappropriate spectral filter, or perform any other spectralprogramming/control activity. In some embodiments, the spectralcontroller 106 includes a DLP micro-mirror array (e.g. TEXAS INSTRUMENTSDLP5500) enabling configuration of spectral structures or selection ofpredetermined spectral structures by activating or deactivatingspecified array elements. The programmable nature of the spectralcontroller 106 enables access to an increased number of spectralstructures defined by predetermined and/or customized configurations.For example, the spectral configurations may be loaded from storagemedia and/or determined according to user input or specified accordingto system, device, or sample parameters.

FIGS. 2A and 2B illustrate an embodiment of a spectral control system200 including multiple sub-dispersion paths and sub-combination paths toenable wider spread of illumination spectra. Broadband illuminationdelivered along the dispersion path by an illumination source 202 may besplit into a selected number of portions, each within a respectivespectral band. Then each portion may be dispersed into a plurality ofdispersed portions forming a plurality of spectral continuums (as shownin FIG. 2B) for higher resolution control of the illumination spectra.For example, illumination may be split among six sub-dispersion pathsleading to six respective dispersive elements 206 for as much as sixtimes the resolution attainable with a single-line dispersion path. Theforegoing example is illustrative of the increased resolution ofspectral control attainable with multi-path dispersion. It is noted,however, that the number of sub-paths is arbitrary and no limitation isintended by the illustrative examples provided herein.

In some embodiments, the dispersion path may include a first pluralityof dichroic splitters 204 configured to receive illumination directedalong the dispersion path from the illumination source 202. The dichroicsplitters 204 may be further configured to direct portions illuminationwithin specified spectral bands or ranges along sub-dispersion pathsleading to the respective dispersive elements 206. For example, thedichroic splitters 206 may be configured to divide illumination receivedfrom the illumination source 202 into a plurality of bands within therange of 300 to 900 nm. The dispersion path may further include a secondplurality of dichroic splitters 208 configured to direct the dispersedportions of illumination from each sub-dispersion path along a commonpath to at least one spectral controller 210.

FIG. 2B illustrates the dispersed portions of illumination from eachsub-path directed onto a surface of the spectral controller 210. Bydividing the illumination into a plurality of portions within respectivespectral bands and then dispersing each portion into a spread outspectral continuum, the spectral controller 210 is enabled to excludeselected portions within each of the plurality of spectral bands. Sincethe illumination is spread over a greater surface (e.g. six times asmuch surface area) of the spectral controller 210, the spectralcontroller 210 is enabled to control the excluded/transmitted selectionof illumination spectra with improved resolution due to the increasenumber of array elements that can be activated/deactivated to affectillumination spectra. Higher resolution without undue cost and systemcomplexity may be achieved by separating illumination into multiplestrips directed at a single spatial light modulator. Furthermore,recombining the spectrally controlled illumination along a commonillumination path preserves spectral brightness of the illuminationsource 202.

As shown in FIG. 2A, the combination path may include a reversedconfiguration of elements that mirrors the dispersion path. For example,the combination path may include a first plurality of dichroic combiners212 configured to direct transmitted portions of dispersed illuminationfrom the spectral controller 210 along a plurality of sub-combinationpaths leading to respective combination elements 214. The combinationpath may further include a second plurality of dichroic combiners 216configured to direct the substantially coaxial (undispersed)illumination received from each combination element 214 (i.e. eachsub-combination path) onto a common illumination path. In someembodiments, the illumination path may further include one or moreneutral density filters 218 configured to control an intensity level ofthe spectrally controlled illumination. As discussed above, theillumination path may be delineated by any number of optical elements,such as an optical fiber 220 configured to deliver at least a portion ofspectrally controlled illumination to an optical measurement head oranother output source.

It is noted that the terms “dichroic splitter” and “dichroic combiner”may be interchangeably utilized to reference an illumination splittingor combining functionality. However, the term “dichroic splitter” isgenerally used herein to refer to a dichroic splitter/combiner disposedalong the dispersive path, and the term “dichroic combiner” is generallyused herein to refer to a dichroic splitter/combiner disposed along thecombination path. Accordingly, the use of either term should not beunderstood to limit the disclosure in any way.

FIG. 3 illustrates an embodiment of a spectral control system 300including multiple spectral control paths, where selection of a controlpath determines spectral attributes of output illumination. The system300 may include at least one illumination source 302, such as sources202 or 102 described above. The illumination source 302 may beconfigured to provide illumination along a switching path defined by oneor more optical elements or free space to an optical switch 304A. Insome embodiments, the optical switch 304A includes a switching mirrorassembly, such as an actuator (e.g. motor or servo) coupled to a mirror,a galvanometer (“galvo”) mirror, or galvo scanner. The optical switch304A may be configured to direct at least a portion of illuminationreceived from the illumination source 302 along a selected spectralcontrol path. For example, a first spectral control path and a secondspectral control path are illustrated following the optical switch 304Ain FIG. 3. It is noted, however, that any number (i.e. two or more) ofspectral control paths may be employed without substantial deviationfrom the architecture described herein.

In some embodiments, the system 300 includes fold mirrors 306A-306D orother optical elements defining the spectral control paths to reduceoverall system footprint by allowing flexibility in disposition of onespectral control path relative to another. For example, the opticalswitch 304A may be configured to direct illumination along a selectedone of a first spectral control path defined by fold mirrors 306A and306C or a second spectral control path defined by fold mirrors 306B and306D. In some embodiments, the system 300 further includes a secondoptical switch 304B working in parallel with the first optical switch304A. The second optical switch 304B may be configured to receivespectrally controlled illumination from the selected spectral controlpath and further configured to direct the spectrally controlledillumination along an illumination path. Other optical switchingarrangements may be employed without departing from the scope of thisdisclosure. For instance, a single optical switch 304 may be configuredto direct illumination from the illumination source 302 along a selectedspectral control path, where the output spectrally controlledillumination is directed along the illumination path by an arrangementof optical elements known to the art, such as any combination of prisms,lenses, optical fibers, or the like.

Each spectral control path may include at least one spectral controller308 configured to block a portion of illumination including an excludedselection of illumination spectra. Each spectral controller 308 may befurther configured to direct a portion of illumination including atransmitted selection of illumination spectra along the illuminationpath, either directly or via optical elements defining a remainder ofthe respective spectral control path. The spectral controllers 308 mayinclude a plurality of selectable filters. For example, each spectralcontroller 308 may include a color wheel or another actuatable structuresupporting a plurality of predefined filters, where a filter is selectedor deselected by actuation within or out of the respective spectralcontrol path. Alternatively, each spectral controller 308 may includeone of a plurality of filters selected by manual disposition within areceiving slot.

The optical switch 304 may enable rapid switching between a plurality ofpreconfigured spectral control paths, such as a first path configuredaccording to a first spectral controller 308A and a second pathconfigured according to a second spectral controller 308B. Further, whenone spectral control path is selected, the other spectral control pathor paths may be reconfigured. For example, the second spectralcontroller 308B may be reconfigured while the first spectral controller308A is active. In some embodiments, for added flexibility of spectralcontrol and increased switching capability, each spectral controller 308may include a highly configurable sub-spectral control system, such assystem 100 or system 200 described above.

The system 300 may further include one or more neutral density filters310 configured to control intensity of the output spectrally controlledillumination. A neutral density filter 310 may be disposed along theillumination path or respective neutral density filters 310A and 310Bmay be disposed in each spectral control path. In some embodiments, thesystem 300 further includes one or more optical elements defining theillumination path, such as an optical fiber 312 (e.g. multi-mode opticalfiber) which may be fed by a coupling lens.

With regards to system 300 as well as systems 100 and 200, the outputspectrally controlled illumination may be fed into an opticalmeasurement head for an optical metrology, inspection, or analysissystem. In some embodiments, the optical measurement head may furtherinclude or may be integrated with an output end of the optical fiber 312(also applicable to optical fibers 110 or 220) transferring at least aportion of the output spectrally controlled illumination.

FIG. 4 illustrates an optical metrology system 400, such as an angularor spectroscopic scatterometry system, configured to determine defectcharacteristics or spatial attributes of at least one sample 402, suchas a semiconductor wafer or mask. Optical metrology systems are wellknown in the art. The following description is illustrative of anembodiment; however, those skilled in the art will appreciate thatconcepts described herein may be extended to alternative embodiments ofoptical metrology or inspection systems without departing from the scopeof this disclosure.

In an embodiment, the system 400 may include a stage 404 configured tosupport a sample 402. The stage 404 may include or may be coupled to atleast one actuator configured to translate or rotate the stage 404.Accordingly, the stage 404 may be configured to support the sample 402at a selected position for receiving illumination delivered along anillumination path to a selected region of the sample surface. In someembodiments, the system 400 is further configured for scanningillumination along a surface of the sample 402.

The system 400 may include at least one illumination source 406 feedinginto a spectral control system 408, such as system 100, system 200, orsystem 300 described above. The spectral control system 408 may beconfigured to deliver illumination via free space coupling or fibercoupling via an optical fiber 410 to an optical measurement head 412 ofthe system 400. As discussed above with regard to some embodiments, theoptical measurement head 412 may further include or may be integratedwith the output end of the optical fiber 410. The optical measurementhead 412 may be configured to provide at least a portion of thespectrally controlled illumination along the illumination path toilluminate a surface of the sample 402.

In some embodiments, the optical measurement head 412, in accordancewith the spectral control system 408, is configured to illuminatedifferent portions (e.g. semiconductor device layers) of the sample 402with portions of illumination having different spectral attributes. Forexample, the optical measurement head 412 may be configured toilluminate a first portion (e.g. process layer) of the sample 402 withillumination including a first selection of spectra and a second portion(e.g. resist layer) of the sample 402 with illumination including asecond selection of spectra. The transmitted selection of illuminationspectra utilized to illuminate the sample or different layers of thesample may be selected according to various sample, layer, and/ormeasurement characteristics. For example, the transmitted selection ofillumination spectra may be selected and controlled according to overlaysensitivity or a predetermined or predicted level of accuracy. In someembodiments, different measurement recipes are tested over time. Assuch, the transmitted selection of illumination spectra may be basedupon at least one preceding measurement.

Illumination optics including, but not limited to, a beam splitter 414and an objective lens 416 may be disposed along the illumination path.For example, the beam splitter 414 may be configured to direct at leasta portion of the illumination through the objective lens 416 to thesample surface. Illumination scattered, reflected, or radiated by thesample 402 is directed along a collection path delineated by collectionoptics to at least one detector 418, such as a spectrometer, camera, orany other photodetector. The system may further include a computingsystem 420 communicatively coupled to the detector 418. The computingsystem 420 may be configured to determine at least one spatial attributeof the sample or defect information based upon the detected illuminationin accordance with a metrology or detection algorithm embedded inprogram instructions 424 executed from at least one carrier medium 422.In some embodiments, the computing system 420 is further configured todrive the spectral control system 408. For example, the computing system420 may be configured to drive components of the spectral control system408, such as a spectral controller, optical switch, or the like.

In some embodiments, the system 400 is configured for spectroscopicscatterometry where the detector 418 includes a simple detectorconfigured for integrating the signal received according to apredetermined controlled illumination spectrum. In some embodiments, thesystem 400 is configured for combined angular and spectroscopicscatterometry where the detectors 418 include a set of fiberspectrometers with inputs mounted to a pupil of the system 400 foranalyzing spectrum and its dependence on pupil position. In someembodiments, the system 400 is configured for combined angular andspectroscopic scatterometry where a plurality of fiber coupled detectors418 with inputs mounted to the pupil are configured scan over thepredetermined controlled spectral range. The foregoing embodiments areillustrative of measurement improvement and increased system capabilityresulting from the enhanced spectral control described herein.

FIG. 5 illustrates a method 500 of controlling spectral attributes ofillumination in accordance with system 100 and/or system 200. As such,method 500 includes steps for executing any of the above describedfunctions or implementing any of the above described features inaddition to the steps that follow. It is noted, however, that one ormore steps of method 500 may be executed via systems or configurationbeyond the embodiments of systems 100 and 200 described above. Method500 should be construed to encompass any system or device configured tocarry out the following steps.

At steps 502 and 504, illumination directed along a dispersion path isdispersed by one or more dispersion elements and further directed to aspectral controller. In some embodiments, the dispersion path includes afirst plurality of splitters/combiners configured divide theillumination among a selection of spectral ranges, each portion beingdirected along a sub-dispersion path to a respective dispersion element.The dispersion path may further include a second plurality ofsplitters/combiners configured to receive dispersed portions ofillumination from the plurality of dispersion elements. The dispersedportions of illumination may be directed from the second plurality ofsplitters/combiners to the spectral controller such that each selectionof the dispersed portions is delivered forming a spectral continuumacross the surface of the spectral controller, as illustrated in FIG. 1C(single-path) and FIG. 2B (multi-path).

At steps 506 and 508, the illumination spectrum controlled by blockingselected portions of the dispersed illumination from being transmittedalong a combination path. In some embodiments, the spectral controllerexcludes selected portions by activating/deactivating array elements oropening/closing active shutters. Accordingly, only selected portion ofthe spectrally dispersed illumination, hence only a selected portion ofthe illumination spectra, are transmitted along the combination path.

At step 510, the transmitted portions of dispersed illumination arerecombined into substantially coaxial or undispersed illuminationutilizing one or more combination elements. In some embodiments, forexample, the combination elements may be similar to the dispersiveelements in structure but arranged for functionally reversing theoperation of the dispersive elements on the transmitted portions ofillumination. At step 512, the undispersed (spectrally controlled)illumination is directed along an illumination path. For example, thespectrally controlled illumination may be directed through a couplinglens feeding at least a portion of the illumination into an opticalfiber coupled to an optical measurement head.

FIG. 6 illustrates a method 600 of controlling spectral attributes ofillumination in accordance with system 300. As such, method 600 includessteps for executing any of the above described functions or implementingany of the above described features in addition to the steps thatfollow. It is noted, however, that one or more steps of method 600 maybe executed via systems or configuration beyond the embodiments ofsystem 300 described above. Method 600 should be construed to encompassany system or device configured to carry out the following steps.

At step 602, at least a portion of illumination emanating from anillumination source 302 is directed along a selected path of a pluralityof spectral control paths via at least one optical switch 304. In someembodiments, a first optical switch 304A operates in parallel with asecond optical switch 304B. For example, the first optical switch 304Amay direct illumination along the selected spectral control path, andthe second optical switch 304B may direct spectrally controlledillumination received from the selected spectral control path along anillumination path.

At step 604, a spectral controller 308 disposed within the selectedspectral control path blocks a first portion of illumination includingan excluded selection of illumination spectra. At step 606, the secondportion of illumination including a transmitted selection ofillumination spectra is directed from the spectral controller 308 alongthe illumination path, either directly or indirectly (e.g. via thesecond optical switch 304B). Further, an intensity level of thespectrally controlled illumination may be adjusted via one or moreneutral density filters disposed within each spectral control path ordisposed within the illumination path. In some embodiments, thespectrally controlled illumination is further directed along an opticalfiber feeding into an optical measurement head or another output source,as discussed above.

FIG. 7 illustrates a method 700 of performing optical metrologyutilizing spectrally controlled illumination in accordance with system400. As such, method 700 includes steps for executing any of the abovedescribed functions or implementing any of the above described featuresin addition to the steps that follow. It is noted, however, that one ormore steps of method 700 may be executed via systems or configurationbeyond the embodiments of system 400 described above. Method 700 shouldbe construed to encompass any system or device configured to carry outthe following steps.

At step 702, at least one spectral attribute of illumination emanatingfrom an illumination source 406 is controlled via a spectral controlsystem 408. For example, the spectral control system 408 may execute thesteps of method 500 or method 600 to provide spectrally controlledillumination including a transmitted selection of illumination spectraalong an illumination path (e.g. via optical fiber 410) to an opticalmeasurement head 412. In some embodiments, the transmitted selection ofillumination spectra (i.e. the spectral attributes of the illumination)are selected according to overlay sensitivity, measurement accuracy,and/or various sample characteristics. In some embodiments, the selectedspectral attributes are based upon one or more preceding measurements.For example, different measurement recipes may be tested over time toselect an optimal or near optimal spectral configuration.

Various portions of a metrology sample 402, such as a semiconductorwafer, mask, or patterned target structure, may respond differently toillumination spectra. For example, a first portion (e.g. process layer)and a second portion (e.g. resist layer) may be more or less sensitiveto certain portions of an illumination spectrum. It may be advantageousto analyze each portion of the sample 402 utilizing a different spectralconfiguration. As such, some embodiments may include steps 704 and 706of illuminating a first portion of the sample 402 utilizing illuminationincluding a first selection of illumination spectra and a second portionof the sample 402 utilizing illumination including a second selection ofillumination spectra.

At step 708 and 710, illumination scattered, reflected, or radiated fromthe sample 402 is detected, and at least one spatial attribute of thesample 402 is determined utilizing information (e.g. intensity,waveform, polarity, spectral content, image content) associated with thedetected illumination. In some embodiments, steps 708 and 710 arerepeated for each portion (e.g. device layer) of the sample 402 analyzedutilizing different illumination spectra. Accordingly, different samples402 or different portions of the same sample 402 can be analyzed withimproved sensitivity by illuminating each sample 402 or sample portionwith illumination including individually selected illumination spectra.

Those having skill in the art will further appreciate that there arevarious vehicles by which processes and/or systems and/or othertechnologies described herein can be effected (e.g., hardware, software,and/or firmware), and that the preferred vehicle will vary with thecontext in which the processes and/or systems and/or other technologiesare deployed. Program instructions implementing methods such as thosedescribed herein may be transmitted over or stored on carrier media. Acarrier medium may include a transmission medium such as a wire, cable,or wireless transmission link. The carrier medium may also include astorage medium such as a read-only memory, a random access memory, amagnetic or optical disk, or a magnetic tape.

All of the methods described herein may include storing results of oneor more steps of the method embodiments in a storage medium. The resultsmay include any of the results described herein and may be stored in anymanner known in the art. The storage medium may include any storagemedium described herein or any other suitable storage medium known inthe art. After the results have been stored, the results can be accessedin the storage medium and used by any of the method or systemembodiments described herein, formatted for display to a user, used byanother software module, method, or system, etc. Furthermore, theresults may be stored “permanently,” “semi-permanently,” temporarily, orfor some period of time. For example, the storage medium may be randomaccess memory (RAM), and the results may not necessarily persistindefinitely in the storage medium.

Although particular embodiments of this invention have been illustrated,it is apparent that various modifications and embodiments of theinvention may be made by those skilled in the art without departing fromthe scope and spirit of the foregoing disclosure. Accordingly, the scopeof the invention should be limited only by the claims appended hereto.

What is claimed is:
 1. A system for controlling at least one spectral attribute of illumination, comprising: one or more dispersive elements disposed along a dispersion path, the one or more dispersive elements configured to receive at least a first portion of illumination directed along the dispersion path from at least one illumination source, and further configured to disperse the first portion of the illumination into a first plurality of dispersed portions of illumination; at least one spectral controller configured to receive the first plurality of dispersed portions of illumination from the dispersion path, further configured to block an excluded selection of the first plurality of dispersed portions of illumination, and further configured to direct a transmitted selection of the first plurality of dispersed portions of illumination along a combination path; and one or more combination elements disposed along the combination path, the one or more combination elements configured to combine the transmitted selection of the first plurality of dispersed portions of illumination into substantially coaxial illumination directed along an illumination path.
 2. The system of claim 1, wherein the one or more dispersive elements include one or more diffraction gratings.
 3. The system of claim 1, wherein the one or more combination elements include one or more diffraction gratings.
 4. The system of claim 1, wherein the one or more dispersive elements include one or more prisms.
 5. The system of claim 1, wherein the one or more combination elements include one or more prisms.
 6. The system of claim 1, wherein the at least one spectral controller includes a micro-mirror array.
 7. The system of claim 1, wherein the at least one spectral controller includes a plurality of active shutters.
 8. The system of claim 1, wherein the at least one spectral controller includes a plurality of selectable filters.
 9. The system of claim 1, further comprising: a first plurality of dichroic splitters configured to direct portions of illumination from the at least one illumination source along respective paths to a plurality of dispersive elements; a second plurality of dichroic splitters configured to direct dispersed portions of illumination from the plurality of dispersive elements to the at least one spectral controller; a first plurality of dichroic combiners configured to direct portions of illumination from the at least one spectral controller along respective paths to a plurality of combination elements; and a second plurality of dichroic combiners configured to direct combined portions of illumination from the plurality of combination elements along the illumination path.
 10. The system of claim 1, further comprising: one or more neutral density filters configured to control an intensity level of the substantially coaxial illumination directed along the illumination path.
 11. The system of claim 1, wherein the illumination path includes at least one optical fiber configured to direct the substantially coaxial illumination to an optical measurement head of an optical metrology system.
 12. An optical metrology system, comprising: at least one illumination source; a spectral control system configured to receive illumination emanating from the at least one illumination source, further configured to block a portion of the illumination including an excluded selection of illumination spectra, and further configured to direct a portion of the illumination including a transmitted selection of illumination spectra along an illumination path; an optical measurement head configured to illuminate a sample disposed upon a sample stage utilizing at least a portion of illumination received from the illumination path, the optical measurement head being configured to illuminate at least a first portion of the sample utilizing a first portion of illumination including a first selection of illumination spectra and a second portion of the sample utilizing a second portion of illumination including a second selection of illumination spectra; at least one detector configured to receive illumination scattered, reflected, or radiated from the sample; and at least one computing system communicatively coupled to the at least one detector, the at least one computing system configured to determine at least one spatial attribute of the sample based upon the illumination scattered, reflected, or radiated from the sample.
 13. The system of claim 12, wherein the first portion of the sample includes a process layer, and wherein the second portion of the sample includes a resist layer.
 14. The system of claim 12, wherein the transmitted selection of illumination spectra is based upon at least one of overlay sensitivity and measurement accuracy.
 15. The system of claim 12, wherein the transmitted selection of illumination spectra is based upon at least one preceding measurement.
 16. The system of claim 12, wherein the spectral control system includes: a first plurality of dichroic splitters configured to receive illumination from emanating from the at least one illumination source, and further configured to direct multiple portions of the illumination along a plurality of dispersion paths; a plurality of dispersive elements, each dispersive element located along a respective one of the plurality of dispersion paths; a second plurality of dichroic splitters configured to direct dispersed portions of illumination from the plurality of dispersive elements to at least one spectral controller, the at least one spectral controller configured to block an excluded selection of the dispersed portions of illumination; a first plurality of dichroic combiners configured to receive a transmitted selection of the dispersed portions of illumination from the at least one spectral controller, and further configured to direct the transmitted selection of the dispersed portions of illumination along a plurality of combination paths; a plurality of combination elements, each combination element located along a respective one of the plurality of combination paths; and a second plurality of dichroic combiners configured to direct combined portions of illumination from the plurality of combination elements along the illumination path.
 17. The system of claim 12, wherein the spectral control system includes: a first optical switch configured to receive illumination emanating from the at least one illumination source, and further configured to direct the illumination along a selected path of a plurality of spectral control paths; a plurality of spectral controllers, at least one spectral controller of the plurality of spectral controllers being disposed along each of the plurality of spectral control paths, the at least one spectral controller being configured to block a portion of the illumination including an excluded selection of illumination spectra; and a second optical switch configured to receive a portion of the illumination including a transmitted selection of illumination spectra from the at least one spectral controller of the selected path of the plurality of spectral control paths, and further configured to direct the portion of the illumination including the transmitted selection of illumination spectra along the illumination path.
 18. The system of claim 12, further comprising: one or more neutral density filters configured to control an intensity level of at least a portion of illumination directed along the illumination path.
 19. The system of claim 12, wherein the illumination path includes at least one optical fiber configured to direct at least a portion of illumination to the optical measurement head.
 20. A method of performing optical metrology, comprising: controlling at least one spectral attribute of illumination received from at least one illumination source, including: blocking a portion of the illumination including an excluded selection of illumination spectra and directing a portion of the illumination including a transmitted selection of illumination spectra along an illumination path; illuminating a sample disposed upon a sample stage utilizing at least a portion of illumination received from the illumination path, including: illuminating at least a first portion of the sample utilizing a first portion of illumination including a first selection of illumination spectra and a second portion of the sample utilizing a second portion of illumination including a second selection of illumination spectra; detecting illumination scattered, reflected, or radiated from the sample; and determining at least one spatial attribute of the sample based upon the illumination scattered, reflected, or radiated from the sample.
 21. The method of claim 20, wherein controlling the at least one spectral attribute of the illumination received from the at least one illumination source includes: selecting the transmitted selection of illumination spectra based upon at least one of overlay sensitivity and measurement accuracy.
 22. The method of claim 20, wherein controlling the at least one spectral attribute of the illumination received from the at least one illumination source includes: selecting the transmitted selection of illumination spectra based upon at least one preceding measurement.
 23. The method of claim 20, wherein controlling the at least one spectral attribute of the illumination received from the at least one illumination source includes: directing at least a first portion of illumination from the at least one illumination source along a dispersion path; dispersing the first portion of illumination into a first plurality of dispersed portions of illumination utilizing one or more dispersive elements; blocking an excluded selection of the first plurality of dispersed portions of illumination utilizing at least one spectral controller; directing a transmitted selection of the first plurality of dispersed portions of illumination from the at least one spectral controller along a combination path; combining the transmitted selection of the first plurality of dispersed portions of illumination utilizing one or more combination elements; and directing substantially coaxial illumination including the combined portions of illumination along an illumination path.
 24. The method of claim 23, wherein controlling the at least one spectral attribute of the illumination received from the at least one illumination source further includes: directing portions of illumination from the at least one illumination source along respective paths to a plurality of dispersive elements utilizing a first plurality of dichroic splitters; directing dispersed portions of illumination from the plurality of dispersive elements to at least one spectral controller utilizing a second plurality of dichroic splitters; directing selected portions of illumination from the at least one spectral controller along respective paths to a plurality of combination elements utilizing a first plurality of dichroic combiners; and directing combined portions of illumination from the plurality of combination elements along the illumination path utilizing a second plurality of dichroic combiners.
 25. The method of claim 20, wherein controlling the at least one spectral attribute of the illumination received from the at least one illumination source includes: directing illumination received from the at least one illumination source along a selected path of a plurality of spectral control paths utilizing an optical switch, each spectral control path of the plurality of spectral control paths including at least one spectral controller; blocking the portion of the illumination including the excluded selection of illumination spectra utilizing a respective spectral controller of the selected path of the plurality of spectral control paths; and directing the portion of the illumination including the transmitted selection of illumination spectra along the illumination path.
 26. The method of claim 25, wherein directing the portion of the illumination including the transmitted selection of illumination spectra along the illumination path includes: directing illumination received from the selected path of the plurality of spectral control paths along the illumination path utilizing a second optical switch.
 27. The method of claim 20, further comprising: controlling an intensity level of the substantially coaxial illumination directed along the illumination path utilizing one or more neutral density filters.
 28. The method of claim 20, further comprising: directing the substantially coaxial illumination to an optical measurement head of an optical metrology system utilizing an optical fiber. 